This house is apprx 17 years old. The foundation rests on clay soil. There are no cracks in the sheetrock in any rooms and all windows and doors open without binding. All cracks are adjacent to 2nd floor windows. Could there be any reason other than foundation settling that has caused these cracks/shifting of the bricks?

Looks like simple settling to me. But it does need to be tuck pointed asap before water gets in. Looks like buff colored morter. May have to go to a real concrete supply house to find it.
Those joints also take a special tool to get them to match. Lowes and Home Depot both have them.

The wood frame structure shrinks with time, resulting in a differential movement that can crack mortar joints.

Differential movement is common, but usually not seen in very many 2 story structures. For larger structure made from material more stable than wood (steel, concrete, masonry) there are requirements for the used of "soft" horizontal joints at about every other floor and your home is not far from that limit.

Brick veneer is connected to the structural back-up with flexible ties to allow the movement, but openings and corners are stress concentration points. Wood moves and shrinks more than other materials, so that makes problems more common.

Have the cracks ground out and tuck pointed with a similar colored mortar to minimize moisture intrusion and for cosmetic purposes.

after you have the repairs done, consider applying a silicone siloxane to help the brick last longer,,, its an invisible water repellent altho it will darken the overall appearance SLIGHTLY ( no financial interest )

When you have an opening in a brick wall, you should either build across the opening using a key stone, shaped bricks/mortar to make sufficient support for the weight of the wall above. Or you should use a steel lintel. From the way the mortar and bricks have been laid and have cracked there is no support. If the foundations had cracked, you would be able to follow the cracks down to the foundations.